Battery pack

ABSTRACT

A battery pack includes: a plurality of batteries; a housing for containing the batteries; and at least one partition plate for separating the batteries from one another. The at least one partition plate includes a metal mesh and a heat insulating layer disposed on each side of the metal mesh. The heat insulating layer includes a foam material capable of foaming at a temperature of 110° C. or more and 200° C. or less, so that the thickness of the heat insulating layer increases when the foam material foams. Even when one of the batteries contained in the battery pack generates abnormal heat, the conduction of the heat to other batteries can be effectively suppressed.

TECHNICAL FIELD

This invention relates to a battery pack including a plurality ofbatteries, and particularly to an improvement in the structure of abattery pack.

BACKGROUND ART

Recently, portable appliances such as notebook personal computers andcellular phones have become widely used, and there is thus an increasingdemand for batteries as the power source for portable appliances. Inparticular, there is an increasing demand for secondary batteries thatare small and light-weight, have high energy density, and can berepeatedly charged and discharged.

To meet such demand, non-aqueous electrolyte secondary batteries areunder active research and development. Since non-aqueous electrolytesecondary batteries contain large energy due to an increase in thefunctionality of portable appliances, they generate large amounts ofheat in the event of abnormal conditions.

Thus, proposals have been made on the structure of a battery packcontaining such batteries, in order to suppress the conduction ofabnormal heat, generated by a specific battery due to some reason, toadjacent batteries.

PTL 1 proposes disposing a partition plate with electrical and thermalinsulating properties, made of a resin such as polypropylene orpolycarbonate, between a plurality of batteries. The partition platesuppresses the conduction of heat of a specific battery generatingabnormal heat to adjacent batteries.

In order to make the partition plate more flame-retardant, PTL 2proposes using inorganic refractory materials such as mica and ceramics.

PTL 3 proposes forming a cavity in a partition plate made of aninflammable resin such as polyethylene or polypropylene and filling thecavity with a fire-extinguishing agent such as ammonium dihydrogenphosphate. In the event of abnormal heat generation of a battery, a part(a part with a low melting point) of the partition plate melts due tothe heat, so that an opening is formed in the partition plate. Thefire-extinguishing agent then flows out of the opening, and the emptycavity of the partition plate provides heat insulation.

CITATION LIST Patent Literature

-   PTL 1: Japanese Laid-Open Patent Publication No. 2008-192570-   PTL 2: Japanese Laid-Open Patent Publication No. 2008-218210-   PTL 3: Japanese Laid-Open Patent Publication No. 2009-4362

SUMMARY OF INVENTION Technical Problem

The partition plates of PTLs 1 to 3 need to be somewhat thick uponbattery pack production, thereby resulting in low volumetric efficiency.It is thus difficult to reduce the size of the battery packs.

Also, when a large amount of heat resulting from abnormal heatgeneration of a specific battery in a battery pack is concentrated in aspecific part of a partition plate, the partition plate may be damagedand the heat may be transferred to adjacent batteries.

As such, the invention provides a highly safe, small, and light-weightbattery pack with a high heat insulation effect, so that even when alarge amount of heat is generated in a specific part of the battery packdue to abnormal heat generation of one of batteries contained in thebattery pack, the conduction of the heat to other batteries can beeffectively suppressed.

Solution to Problem

The invention is directed to a battery pack including: a plurality ofbatteries; a housing for containing the batteries; and at least onepartition plate for separating the batteries from one another. The atleast one partition plate includes a metal mesh and a heat insulatinglayer disposed on each side of the metal mesh. The heat insulating layerincludes a foam material capable of foaming at a temperature of 110° C.or more and 200° C. or less, so that the thickness of the heatinsulating layer increases when the foam material foams.

Advantageous Effects of Invention

According to the invention, when the partition plate comprising the heatinsulating layers disposed on both sides of the metal mesh is heated,the foam material in the heat insulating layers foams, thereby producinga large number of gas bubbles. As a result, the heat insulating layersexpand, and the partition plate exhibits good heat insulation. Since thepartition plate includes the metal mesh, the partition plate allows theheat to be dispersed efficiently. Therefore, by disposing the partitionplate between adjacent batteries, even when a large amount of heat isgenerated in a specific part of the battery pack due to abnormal heatgeneration of one of batteries contained in the battery pack, theconduction of the heat to other batteries can be effectively suppressed.Also, since the heat insulating layers are stably held by the metalmesh, the thickness of the partition plate can be reduced. The use ofsuch a partition plate can provide a highly safe, small, andlight-weight battery pack with a high heat insulation effect.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic longitudinal sectional view of a battery pack inone embodiment of the invention; and

FIG. 2 is a sectional view along the line II-II of FIG. 1.

DESCRIPTION OF EMBODIMENTS

The invention relates to a battery pack including a plurality of (two ormore) batteries, a housing for containing the batteries, and at leastone partition plate for separating the batteries from one another.

When a plurality of tubular batteries are placed in a housing in such amanner that the side faces of the adjacent batteries face one another,the at least one partition plate is disposed between at least the sidefaces of the adjacent batteries.

The tubular batteries can be, for example, cylindrical or prismatic. Acylindrical battery or a prismatic battery is in the form of a cylinderor a quadrangular prism having a positive terminal at one end face and anegative terminal at the other end face. When a plurality of cylindricalor prismatic batteries are aligned side by side in such a manner thatthe side faces of the adjacent batteries face one another, a partitionplate is disposed between the side faces. When the end faces of adjacentbatteries face one another, a partition plate may or may not be disposedbetween the end faces.

In the invention, the partition plate has the following features (1) to(3).

(1) The partition plate comprises a heat conductive layer and a heatinsulating layer disposed on each side of the heat conductive layer.

(2) The heat conductive layer is a metal mesh.

(3) Each heat insulating layer includes a foam material capable offoaming at a temperature of 110° C. or more and 200° C. or less(hereinafter “first temperature”), and the thickness of the heatinsulating layer increases when the foam material foams.

The partition plate included in the battery pack of the inventioncombines a metal mesh and heat insulating layers containing a foammaterial, thereby making it possible to diffuse and absorb heateffectively with good balance and improve the safety of the battery packsignificantly. Specifically, heat is effectively absorbed by the twoheat insulating layers, and heat is effectively dispersed by the metalmesh. As a result, the conduction of heat of a battery generatingabnormal heat to adjacent batteries can be suppressed. Even when a largeamount of heat is concentrated in a specific part of the partitionplate, the partition plate is not damaged, and the conduction of theheat to adjacent batteries can be suppressed reliably.

The heat insulating layer includes a material that foams at the firsttemperature. As used herein, the first temperature refers to thetemperature of the heat insulating layer heated when a battery generatesabnormal heat. In view of the ambient environment of battery packs andthe temperatures of batteries in the event of abnormal heat generation,it is necessary to use a foam material capable of foaming at 110° C. ormore. Silicates of alkali metals, which will be described below, foam attemperatures of 110° C. or more and less than 200° C. In order to makethe temperature of the heat insulating layer equal to or lower than thetemperature of the battery generating heat, the first temperature ispreferably 150° C. or less.

In normal conditions, since the foam material does not foam, the heatinsulating layer does not expand, thus being a thin layer. In the eventthat one of the batteries contained in the battery pack generatesabnormal heat, the heat insulating layer in contact with or adjacent tothat battery is heated, so that the foam material foams. As a result, alarge number of gas bubbles are produced in the heat insulating layer,and the thickness of the heat insulating layer increases. The largenumber of gas bubbles provides good heat insulation, thereby making itpossible to effectively suppress the conduction of the heat to adjacentnormal batteries.

Even when a large amount of heat is concentrated in a specific part ofthe partition plate, as in the case of ejection of hot gas from abattery generating abnormal heat, the heat can be efficiently dispersedby the metal mesh, and thus the partition plate is not damaged.

Since the heat conductive layer is a mesh, heat can be efficientlydispersed by the partition plate, and concentration of heat in aspecific part can be effectively suppressed by the partition plate.

Since the heat conductive layer is a metal mesh and the heat insulatinglayers are stably held by the metal mesh, the partition plate can bemade thin. It is thus possible to reduce the size and weight of thebattery pack.

The partition plate comprising the metal mesh and the heat insulatinglayers including the foam material is generally inexpensive,light-weight, and easy to produce, compared with partition platescomposed mainly of inorganic refractory materials such as mica andceramics, thereby making it possible to reduce the weight and cost ofthe battery pack with a high heat insulation effect.

Also, according to the invention, a foam material is used as a materialof the heat insulating layer. Thus, even in the case of using aninorganic refractory material, the use of only a small amount of theinorganic refractory material can provide a sufficient heat insulationeffect.

An embodiment of the invention is hereinafter described with referenceto drawings, but the invention is not to be construed as being limitedto the following embodiment.

As illustrated in FIGS. 1 and 2, a battery pack 1 of this embodimentincludes a battery 3 and a battery 4, which are cylindrical secondarybatteries, a prismatic resin housing 2 for containing the batteries 3and 4, and a partition plate 5 disposed between the batteries 3 and 4.The partition plate 5 comprises a sheet-like metal mesh 6 and heatinsulating layers 7 a and 7 b disposed on both sides of the metal mesh6. The heat insulating layer 7 a is disposed on the battery 3 side,while the heat insulating layer 7 b is disposed on the battery 4 side.Since the heat insulating layers 7 a and 7 b are disposed on both sidesof the metal mesh 6, even if either one of the batteries 3 and 4generates abnormal heat, the heat can be absorbed efficiently. Besides,the battery pack also includes parts (not shown) necessary for thebattery pack, such as parts for electrically connecting the batteries 3and 4 (e.g., leads) and parts for delivering electricity from thebattery pack to outside (e.g., external terminals). These parts may beselected as appropriate from those conventionally used in battery packs.

Each of the batteries 3 and 4 is a cylinder having a positive terminalat one end face thereof and a negative terminal at the other end face.The batteries 3 and 4 are oriented in the same direction, with theirside faces facing each other. The partition plate 5 is disposed betweenthe side faces of the batteries 3 and 4. Since the end faces of thebatteries 3 and 4 do not face each other, heat conduction is unlikely tooccur between the end faces of the batteries 3 and 4.

The metal mesh 6 preferably comprises at least one selected from thegroup consisting of stainless steel, iron, nickel, aluminum, titanium,and copper. Among them, stainless steel is particularly preferable interms of the strength of the metal mesh, reduction in the size andweight of the battery pack, and costs.

Even when a large amount of heat is concentrated in a specific part ofthe partition plate (heat insulating layer), as in the case of ejectionof hot gas from a battery generating abnormal heat, the heat can beefficiently absorbed and dispersed. Therefore, the damage of thepartition plate 5 due to concentration of a large amount of heat in aspecific part can be suppressed.

In terms of the ability of the metal mesh to hold the heat insulatinglayers and the strength thereof, the metal mesh 6 is preferably of 5 to65 mesh. The metal mesh is, for example, a wire mesh having a largenumber of meshes (openings) made of a metal wire. Examples of wiremeshes include woven wire meshes such as plain woven wire mesh, twillwoven wire mesh, and hexagonal wire mesh. In order to make the heatdiffusion of the metal mesh uniform in the plane direction thereof, itis preferable to make the diameter of the wire of the wire mesh uniformand make the shape and size of the meshes uniform. In terms of the heatdiffusion and strength of the metal mesh, the diameter of the wire ispreferably 0.02 to 0.7 mm. The mesh shape can be polygons such asquadrangles including a square, a rectangle, and a rhombus, and ahexagon (turtleback shape). In terms of the heat diffusion of the metalmesh and the ability of the metal mesh to hold the heat insulatinglayers, the mesh shape is preferably a quadrangle. In terms of the heatdiffusion of the metal mesh and the ability to hold the heat insulatinglayers, the mesh size (opening) is preferably 0.02 to 4.38 mm. In termsof the heat diffusion of the metal mesh and the ability to hold the heatinsulating layers, the open area ratio of the metal mesh is preferably25 to 75%. Also, expanded metal may be used as the metal mesh.

In the case of using batteries with relatively high capacities, such asnon-aqueous electrolyte secondary batteries using negative electrodeactive materials capable of being alloyed with Li, the batterytemperature becomes very high when these batteries generate abnormalheat. When such a battery is used, the metal mesh 6 is preferably formedof a material with a high melting point, such as stainless steel ortitanium.

The heat insulating layers 7 a and 7 b mainly contain a silicate of analkali metal. The alkali metal is preferably at least one selected fromthe group consisting of sodium (Na), potassium (K), and lithium (Li),since it is inexpensive and convenient for producing a silicate.

In terms of binding strength, Na>K>Li. In terms of water resistance,Li>K>Na. Silicates of these three alkali metals can be used singly or incombination, depending on the device for which the battery pack is used.

Alkali metal silicates contain large amounts of water ofcrystallization. Alkali metal silicates containing water ofcrystallization have a composition represented by, for example, theformula M₂O.nSiO₂.xH₂O. In the formula, M is at least one selected fromthe group consisting of Na, K, and Li. When M is Na or Li, n is 0.5 to4. When M is K, n is 0.4 to 4. x is a value representing the amount ofwater of crystallization, and can be any value depending on the amountof water of crystallization. For example, in the case of sodiummetasilicate, M=Na and n=1. In the case of potassium metasilicate, M=Kand n=1. In the case of lithium metasilicate, M=Li and n=1.

When an alkali metal silicate containing water of crystallization isexposed to a high temperature, the alkali metal silicate starts torelease water of crystallization around approximately 110° C., and atthe same time, starts to foam. As such, a large number of gas bubblesare produced inside the heat insulating layers 7 a and 7 b, so thattheir thicknesses increase to provide improved heat insulation.

For example, in the case of abnormal heat generation of the battery 3 inthe battery pack 1, when the heat insulating layers 7 a and 7 b areheated to the first temperature, the alkali metal silicate in the heatinsulating layers 7 a and 7 b foams, so the heat insulating layers 7 aand 7 b expand while producing gas bubbles. As a result, the expandedheat insulating layers 7 a and 7 b containing a large number of the gasbubbles suppress the conduction of the heat from the battery 3 to theadjacent battery 4, thereby preventing the abnormal heat generation ofthe battery 3 from affecting the battery 4.

The heat of the battery 3 generating abnormal heat is sequentiallytransferred to the heat insulating layer 7 a, the metal mesh 6, and theheat insulating layer 7 b. Since the heat transferred to the metal mesh6 from the heat insulating layer 7 a diffuses throughout the partitionplate, the heat can be efficiently absorbed by the heat insulating layer7 b. Even when a large amount of heat is transferred to a specific partof the partition plate 5, as in the case of ejection of hot gas from thebattery 3 generating abnormal heat, the heat is dispersed in the mealmesh 6, and thus the damage of the partition plate due to concentrationof a large amount of heat in the partition plate can be suppressed.

In terms of the space inside the battery pack 1 and the heat insulationof the partition plate 5, the rate of expansion of the heat insulatinglayers 7 a and 7 b in the thickness direction is preferably 30 to 600%,and more preferably 50 to 300%.

The expansion rate is represented by the following formula.

Expansion rate (%)=(thickness of heat insulating layer afterexpansion−thickness of heat insulating layer beforeexpansion)/(thickness of heat insulating layer before expansion)×100

The degree of expansion of the heat insulating layer can be adjustedaccording to, for example, the kind and content of the foam material andthe content of a foam promoter which will be described below. Thethickness of the heat insulating layer refers to the thickness in thethickness direction of the partition plate.

Further, the heat insulating layers 7 a and 7 b also have the effect ofcooling the batteries due to latent heat when the alkali metal silicatereleases water of crystallization. Therefore, an alkali metal silicateis very preferable as a material of the heat insulating layer. When thefoam material is an alkali metal silicate, the cooling effect can beobtained in addition to the heat insulation effect obtained by foamingof the foam material, and thus, the conduction of the heat generated bythe battery 3 to the battery 4 can be suppressed more effectively.

Also, since the partition plate 5 is composed mainly of an incombustiblematerial having no ignition point or flash point, it is suitable forenhancing the reliability of the battery pack 1. In the partition plate5, both sides of the metal mesh are covered with the heat insulatinglayers having an electrical insulating property, and thus, the partitionplate 5 does not cause the batteries to externally short-circuit.

In order to provide more effective heat insulation when the heatinsulating layer is heated to a temperature higher than the firsttemperature at which the alkali metal silicate foams, it is preferablethat the heat insulating layers 7 a and 7 b further contain a foampromoter capable of foaming at a temperature of 200° C. or more(hereinafter “second temperature”).

The foam promoter is preferably a material which releases a gas at atemperature higher than the temperature at which the alkali metalsilicate foams by releasing water of crystallization which becomessteam. The foam promoter releases a gas when the heat insulating layeris heated to a temperature higher than the first temperature due toabnormal heat generation of a battery. Thus, the amount of gas whichcontributes to the foaming of the heat insulating layers 7 a and 7 bincreases. Hence, the thickness of the heat insulating layers 7 a and 7b after the foam material has foamed can be increased, compared withthat of the heat insulating layer containing no foam promoter. As aresult, the heat insulation effect of the heat insulating layers 7 a and7 b can be further enhanced.

It is more preferable to use, as the foam promoter, at least oneselected from the group consisting of aluminum hydroxide, calciumhydroxide, magnesium hydroxide, alum, sodium sulfate, calcium carbonate,magnesium carbonate, and barium carbonate. It is preferable to select,as the form promoter, a material which releases a gas at a temperature(second temperature) higher than the temperature at which the alkalimetal silicate foams by releasing water of crystallization whichprovides a gas.

As a representative combination of a foam material and a foam promoter,a sodium silicate in combination with aluminum hydroxide or magnesiumhydroxide is selected. When a sodium silicate is heated to approximately130 to 150° C., it foams by releasing water of crystallization whichbecomes steam. On the other hand, when aluminum hydroxide is heated toapproximately 200 to 300° C., it is thermally decomposed to producesteam. Also, when magnesium hydroxide is heated to approximately 400° C.or more, it is thermally decomposed to produce steam. With such acombination, when the temperature of the heat insulating layer becomeshigher than the temperature at which the sodium silicate releases steam,aluminum hydroxide or magnesium hydroxide, which is a foam promoter, isthermally decomposed to produce steam. Thus, even when the temperatureof the heat insulating layer becomes higher than the temperature atwhich the sodium silicate releases steam, gas bubbles can becontinuously produced in the heat insulating layer.

The content Wa of the foam promoter in the heat insulating layers 7 aand 7 b is preferably 5 to 95 parts by mass, and more preferably 20 to80 parts by mass, per 100 parts by mass of the alkali metal silicate(excluding water of crystallization).

By setting the content Wa of the foam promoter in the heat insulatinglayers 7 a and 7 b to 5 parts by mass or more per 100 parts by mass ofthe alkali metal silicate (excluding water of crystallization), the foampromoter can produce a sufficient effect. By setting the content Wa ofthe foam promoter in the heat insulating layers 7 a and 7 b to 95 partsby mass or less per 100 parts by mass of the alkali metal silicate(excluding water of crystallization), the ratio of the foam material canbe made sufficient. Thus, the heat insulating layers 7 a and 7 b canprovide a sufficient heat insulation effect. Also, in the case of usinga structural material which will be described below, the ratio of thestructural material can be made sufficient, and the adhesion of the heatinsulating layers 7 a and 7 b can be made sufficient. As a result,partial separation of the heat insulating layers 7 a and 7 b from themetal mesh 6 can be prevented.

To improve the morphological stability of the heat insulating layers ina high-temperature environment, it is preferable that the heatinsulating layers 7 a and 7 b further include a structural materialcomprising inorganic particles that do not foam at the secondtemperature. More preferably, the heat insulating layers 7 a and 7 binclude both a foam promoter and a structural material.

The inorganic particles are uniformly dispersed in the heat insulatinglayer. The inorganic particles are preferably particles of a ceramic interms of heat resistance and the morphological stability of the heatinsulating layer.

It is more preferable to use, as the ceramic, at least one selected fromthe group consisting of aluminum silicate, sodium silicofluoride,bentonite, monmorillonite, kaolinite, mullite, diatomaceous earth,alumina, silica, mica, titanium oxide, vermiculite, pearlite, Maglite,sepiolite, talc, calcium silicate, magnesium silicate, calcium sulfate,and cement.

The shape of the particles is, for example, a sphere, a flake, or afiber. When the structural material is in the form of a fiber, itpreferably has a mean fiber length of 0.1 to 100 μm and a mean fiberdiameter of 0.01 to 10 μm. When the structural material is spherical, itpreferably has a mean particle diameter of 0.1 to 100 μm. When thestructural material is in the form of a flake, it preferably has, forexample, a thickness of 0.01 to 10 μm and a maximum size of 0.05 to 100μm.

The content Wb of the structural material in the heat insulating layers7 a and 7 b is preferably 5 to 70 parts by mass, and more preferably 10to 50 parts by mass, per 100 parts by mass of the alkali metal silicate(excluding water of crystallization).

By setting the content Wb of the structural material in the heatinsulating layers 7 a and 7 b to 5 parts by mass or more per 100 partsby mass of the alkali metal silicate (excluding water ofcrystallization), the thicknesses of the heat insulating layers 7 a and7 b can be made uniform when the heat insulating layers 7 a and 7 bexpand. Thus, the resulting foam layers (the expanded heat insulatinglayers) can provide a sufficient heat insulation effect. By setting thecontent Wb of the structural material in the heat insulating layers 7 aand 7 b to 50 parts by mass or less per 100 parts by mass of the alkalimetal silicate (excluding water of crystallization), the ratios of thefoam material and the foam promoter can be made sufficient. Thus, theheat insulating layers 7 a and 7 b can provide a sufficient heatinsulation effect.

Since the metal mesh 6 has a large number of openings (meshes), itallows the size and weight of the battery pack to be reduced. Also, inthe production of the partition plate, the openings of the metal mesh 6are densely filled with a composition for forming heat insulatinglayers, which will be described below, so that heat insulating layers(not shown) are formed in the openings of the metal mesh 6. The heatinsulating layers formed in the openings allow the heat insulatinglayers 7 a and 7 b to be integrated. Thus, the heat insulating layers 7a and 7 b are stably held by the metal mesh 6. Separation and fall-offof the heat insulating layers 7 a and 7 b from the metal mesh 6 aresuppressed.

Since the openings of the metal mesh 6 are of uniform shape and size,the heat insulating layers can be uniformly filled in the openings.Therefore, in the plane direction of the partition plate 5, the heatdiffusion of the metal mesh and the heat absorption of the heatinsulating layers can be made uniform.

Contrary to this, when the heat conductive layer is a porous material(foam), its pores are not of uniform shape and size. The surface of theporous material has a small open area, and some of the pores do notextend in the thickness direction. Also, the ratio of the pores in theporous material is small. Thus, it is difficult to form heat insulatinglayers in the pores of the porous material densely and stably, and it isdifficult to hold the heat insulating layers on the surfaces of theporous material stably. It is difficult to fill the heat insulatinglayers in the porous material uniformly. Heat diffusion and heatabsorption tend to vary in the plane direction of the porous material.

The thickness of the metal mesh 6 is preferably 0.02 mm to 1 mm. Thethickness of the metal mesh 6 as used herein refers to the largestthickness in a section along the thickness direction of the metal mesh6. When the thickness of the metal mesh is 0.02 mm or more, the heatfrom a battery generating abnormal heat can be effectively absorbed anddispersed. In terms of reducing the size and weight of the battery pack,when the thickness of the metal mesh 6 is 1 mm or less, the size andweight of the battery pack can be readily reduced. The thickness of themetal mesh 6 is more preferably 0.02 to 0.5 mm, and even more preferably0.02 to 0.1 mm.

In terms of the morphological stability of the heat insulating layersand reduction in the size and weight of the battery pack, the thicknessof the heat insulating layer 7 a (the thickness before the foam materialfoams) and the thickness of the heat insulating layer 7 b (the thicknessbefore the foam material foams) are preferably 0.04 to 2 mm, morepreferably 0.04 to 1 mm, and even more preferably 0.04 to 0.5 mm.

The thickness of the partition plate 5 is preferably 0.1 to 5 mm. Thethickness of the partition plate 5 as used herein refers to thethickness before the foam material foams. When the thickness of thepartition plate 5 is 0.1 mm or more, the partition plate 5 can providesufficient heat insulation. When the thickness of the partition plate 5is 5 mm or less, the size and weight of the battery pack can be readilyreduced. The thickness of the partition plate 5 is preferably 0.1 to 2.5mm.

The width of the partition plate 5 (in a plane perpendicular to theaxial direction of the batteries, the length of the partition plate inthe plane direction) is preferably equal to or more than the diameter ofthe batteries 3 and 4 housed in the battery pack (the height of thebatteries from the surface (the inner bottom face of the housing) onwhich the batteries are placed). In this case, the heat insulationeffect of the heat insulating layers 7 a and 7 b is heightened.

The length of the partition plate 5 in the axial direction of thebatteries is preferably equal to or more than the length of thebatteries in the axial direction of the batteries. In this case, theheat insulation effect of the heat insulating layers 7 a and 7 b isheightened.

When prismatic batteries are contained in the housing instead of thecylindrical batteries, the width of the partition plate 5 (in a planeperpendicular to the axial direction of the batteries, the length of thepartition plate 5 in the plane direction) is preferably equal to or morethan the height of the prismatic batteries from the surface (the innerbottom face of the housing) on which the batteries are placed.

The method for producing the battery pack 1 includes, for example, thesteps of:

(A) preparing the housing 2;

(B) preparing a composition containing an alkali metal silicate used toform heat insulating layers;

(C) applying the heat-insulating-layer forming composition prepared instep (B) to both faces of the metal mesh 6 to form layers with a uniformthickness and drying them to form the heat insulating layers 7 a and 7b, thereby producing the partition plate 5; and

(D) placing the battery 3, the battery 4, and the partition plate 5 intothe housing 2 in such a manner the partition plate 5 is disposed betweenthe battery 3 and the battery 4.

Step (A) is described below.

The housing can be produced by, for example, molding a resin. The resinmaterial used to form the housing is preferably a flame-retarded resinclassified as V-0 or higher in UL-94 standard. “A guide to the Safe Useof Lithium Ion Secondary Batteries in Notebook-type Personal Computers”(Japan Electronics Information Industries Association and BatteryAssociation of Japan) recommends the use of such flame-retarded resinsas the resin materials for housings. The constituent material of thehousing is preferably a polymer material which is renderedflame-retardant. The polymer material is preferably one of polycarbonate(PC), polypropylene (PP), polyethylene terephthalate (PET), etc., whichis rendered flame-retardant.

Step (B) is described below.

The composition for forming heat insulating layers can be prepared by,for example, adding a solvent or dispersion medium to an alkali metalsilicate. If necessary, at least one of a foam promoter and a structuralmaterial may be added to the heat-insulating-layer forming composition.The solvent or dispersion medium can be, for example, water or anorganic solvent.

In terms of workability, it is preferable to use liquid glass (anaqueous solution of sodium silicate) as the heat-insulating-layerforming composition. The liquid glass is, for example, sodium silicateNos. 1 to 3 according to JIS (JIS K 1408).

Step (C) is described below.

For example, the heat-insulating-layer forming composition is appliedonto the metal mesh 6 to form coatings, and the coatings are dried toremove the solvent or dispersion medium contained in the coatings, inorder to form the heat insulating layers 7 a and 7 b on the metal mesh6. For example, when liquid glass is used as the heat-insulating-layerforming composition, it is possible to form heat insulating layersincluding solid sodium silicate containing water of crystallization.

The heat-insulating-layer forming composition can be applied by a knowncoating method such as immersion coating, roller coating, sprayingcoating, or doctor blade coating.

In step (C), the composition can be easily filled into the openings ofthe metal mesh in addition to both sides of the metal mesh. It is thuspossible to not only form the heat insulating layers 7 a and 7 b on bothsides of the metal mesh 6 but also form heat insulating layers (notshown) in the openings of the metal mesh 6 easily. As such, the heatinsulating layers 7 a and 7 b can be firmly held by the metal mesh 6.

Step (D) is described below.

In step (D), for example, when the housing 2 prepared in step (A)comprises a case body and a cover, the batteries 3 and 4 are placed inthe case body from the opening of the case body, the partition plate 5is inserted between the batteries 3 and 4, and the cover is attached tothe case body with adhesive or thermal welding. In this manner, thebattery 3, the battery 4, and the partition plate 5 are placed in thehousing 2.

Also, when a groove for receiving an end of the partition plate isformed in a predetermined part of the inner face of the case body instep (A), the end of the partition plate may be fitted to the groove inadvance to place the partition plate in the predetermined position ofthe case body before step (D) of placing the batteries in the case body.

EXAMPLES

Examples of the invention are hereinafter described in details, but theinvention is not to be construed as being limited to these Examples.

To evaluate the safety of the battery pack of the invention, evaluationpacks were produced for evaluation by using metal cylinders instead ofbatteries in the following manner.

Example 1 (1) Preparation of Partition Plate

A composition for forming heat insulating layers was evenly applied ontoboth sides of a nickel mesh (available from Nilaco Corporation,nickel/wire mesh, 20 mesh, part number NI-318020) having a length of 65mm, a width of 20 mm, and a thickness of 0.4 mm, and left for a wholeday and night for natural drying to form heat insulating layers (thethickness of each layer 0.3 mm) comprising sodium silicate containingwater of crystallization. In this manner, a partition plate A (thickness1.0 mm) was prepared. The heat-insulating-layer forming composition wasan aqueous solution of sodium silicate prepared by mixing 80 parts bymass of silicate of soda (available from Osaka Keisou Co., Ltd., tradename: silicate of soda, No. 3) and 20 parts by mass of water. TheNa₂O:SiO₂ molar ratio was 1:3.

(2) Production of Evaluation Pack

Instead of the batteries 3 and 4, two cylinders (length 65 mm and outerdiameter 18 mm) made of SUS 304 were placed in a polycarbonate housingwhose internal space had a length of 67 mm, a width of 41 mm, a depth of20 mm, and a thickness of 1 mm. The partition plate A obtained in theabove manner was disposed between the cylinders in the housing.

Specifically, the housing was composed of a prismatic case body with abottom and a quadrangular cover plate. The two cylinders were placed inthe case body, and the partition plate was inserted between the twocylinders. Thereafter, the case body was fitted with the cover plate. Inthis manner, the partition plate and the two cylinders were placed inthe housing. It should be noted that for an evaluation test describedbelow, a battery pack was produced without joining the case body and thecover plate.

Example 2

Accera coat F (composed mainly of silicate of soda and containing astructural material and the like) available from Access Co., Ltd. wasapplied onto both sides of a stainless steel mesh (available from NilacoCorporation, stainless SUS304/wire mesh, 30 mesh, part number NI-758030)having a length of 65 mm, a width of 20 mm, and a thickness of 0.25 mm,and left for a whole day and night for natural drying to form heatinsulating layers (the thickness of each layer 0.3 mm) comprising sodiumsilicate containing water of crystallization. In this manner, apartition plate B (thickness 0.85 mm) was prepared. The Na₂O:SiO₂ molarratio was 1:3.2.

An evaluation pack was produced in the same manner as in Example 1except for the use of the partition plate B in place of the partitionplate A.

Comparative Example 1

A polycarbonate (PC) plate (length 65 mm, width 20 mm, thickness 1 mm)was prepared as a partition plate C.

An evaluation pack was produced in the same manner as in Example 1except for the use of the partition plate C in place of the partitionplate A.

[Evaluation]

Examples 1 to 2 and Comparative Example 1 were evaluated as follows.

A ceramic heater (MS-M5 available from SAKAGUCHI E. H. VOC CORP.) wasprepared. The cover was detached from the housing, and a plate-like heatgenerator of the ceramic heater was brought into contact with one endface of one of the cylinders, and a pair of lead wires extending fromthe heat generator was connected to a power source having aninter-terminal voltage of 6 V. The temperature of the heater was set to700° C. After ten minutes from the time when the temperature of theheater reached 700° C., the temperature of the other cylinder wasmeasured with a thermocouple.

Also, with respect to Examples 1 and 2, the thickness of the heatinsulating layers before expansion due to heating by the heater and thethickness of the heat insulating layers after expansion due to heatingby the heater were measured using a digital vernier scale. The expansionrate was calculated from the following formula.

Expansion rate (%)=(thickness of heat insulating layer afterexpansion−thickness of heat insulating layer beforeexpansion)/(thickness of heat insulating layer before expansion)×100

Table 1 shows the results. In Table 1, “expansion rate of heatinsulating layer” is the average value of expansion rates of the twoheat insulating layers disposed on both sides of the metal mesh.

TABLE 1 Temperature of Expansion rate of heat cylinder (° C.) insulatinglayer (%) Example 1 110 110 Example 2 106 140 Comparative 320 — Example1

It has been found that the evaluation packs of Examples 1 and 2according to the invention are superior in heat insulation effect to theevaluation pack of Comparative Example 1. It has been found that whenthe partition plates of Examples 1 and 2 are used, the conduction ofheat between adjacent batteries is effectively suppressed, therebyresulting in battery packs with very high safety. In the case of theevaluation pack of Comparative Example 1, the partition plate Cpartially melted and broke. Also, since the partition plate C containsno foam material, it did not expand due to foaming of a foam material,unlike Examples 1 and 2.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

INDUSTRIAL APPLICABILITY

The battery pack according to the invention is useful as the batterypack for personal computers, cellular phones, etc., since the conductionof heat generated by a specific battery due to an abnormal condition toadjacent batteries can be effectively suppressed. It is also applicableto packages for large, stationary batteries, electric vehicle batteries,etc.

1. A battery pack comprising: a plurality of batteries; a housing forcontaining the batteries; and at least one partition plate forseparating the batteries from one another, wherein the at least onepartition plate includes a metal mesh and a heat insulating layerdisposed on each side of the metal mesh, and the heat insulating layerincludes a foam material capable of foaming at a temperature of 110° C.or more and 200° C. or less, so that the thickness of the heatinsulating layer increases when the foam material foams.
 2. The batterypack in accordance with claim 1, wherein the foam material comprises asilicate of an alkali metal containing water of crystallization.
 3. Thebattery pack in accordance with claim 2, wherein the alkali metal is atleast one selected from the group consisting of sodium, potassium, andlithium.
 4. The battery pack in accordance with claim 1, wherein theheat insulating layer further includes: a foam promoter capable offoaming at a temperature of 200° C. or more; and a structural materialcomprising inorganic particles that do not foam at the temperature of200° C. or more.
 5. The battery pack in accordance with claim 4, whereinthe foam promoter is at least one selected from the group consisting ofaluminum hydroxide, calcium hydroxide, magnesium hydroxide, alum, sodiumsulfate, calcium carbonate, magnesium carbonate, and barium carbonate.6. The battery pack in accordance with claim 4, wherein the structuralmaterial is at least one selected from the group consisting of aluminumsilicate, sodium silicofluoride, bentonite, monmorillonite, kaolinite,mullite, diatomaceous earth, alumina, silica, mica, titanium oxide,vermiculite, pearlite, Maglite, sepiolite, talc, calcium silicate,magnesium silicate, calcium sulfate, and cement.
 7. The battery pack inaccordance with claim 1, wherein the metal mesh comprises at least oneselected from the group consisting of stainless steel, iron, nickel,aluminum, titanium, and copper.
 8. The battery pack in accordance withclaim 1, wherein the metal mesh has a thickness of 0.02 mm to 1 mm. 9.The battery pack in accordance with claim 1, wherein the partition platehas a thickness of 0.1 mm to 5 mm before the foam material foams.